The present invention relates to a liquid crystal display device, and more particularly to a thin film transistor--liquid crystal display ("TFT-LCD") device using a light-shielding layer as a capacitor electrode for storing voltage applied to a pixel electrode.
In general, a stagger TFT and an inverted stagger TFT can be used as a switching device in a LCD. The stagger TFT has an advantage in that low resistance of a gate wiring is possible and the number of masks in fabrication process is reduced as compared with the inverted stagger TFT. However, a separate light-shielding layer is needed for preventing degradation of the TFT from backlight.
In active matrix LCD using a TFT array as a switching device for applying a constant signal voltage to a pixel electrode, of fabrication methods of the TFT array having a stagger structure, the technique using three mask patterns which include a mask pattern for a gate line, a mask pattern for a data line and a mask pattern for a light-shielding layer has been practically used.
A layout of mask patterns used in fabricating a conventional TFT-LCD is shown in FIG.1. As shown in this figure, there is a mask pattern 1 for a light-shielding layer, mask pattern 2 for a data line and pixel electrode, and a mask pattern 3 for a gate line, respectively. The mask pattern 2 consists of a pattern 2a for a data line and a pattern 2b for a pixel electrode.
Referring to FIG. 2, a simplified sectional structure of a stagger TFT-LCD which is fabricated by using conventional mask patterns of FIG. 1 is shown. First, an opaque metal layer, for example Cr or Al, is formed to a thickness of about 1000 .ANG. on a transparent insulation substrate 10 and then etched by using the mask pattern 1 for a light shield layer of FIG.1 to form a light-shielding layer 11.
Over the insulation substrate 10 including the light shielding layer 11, an intermediate insulation film 12 is formed. An ITO (indium tin oxide) of a transparent conduction film is formed to a thickness of about 1000 .ANG. on the intermediate insulation film 12 and then etched by using the mask pattern 2 for a data line and pixel electrode to form a data line 13 and a pixel electrode 14. At this time, source/drain electrodes of a TFT (not shown) are formed in the date line and pixel electrode formation step.
Over the glass substrate 10, an amorphous Si layer, an insulation film such as a silicon oxide film, and an Al layer for a gate electrode are formed, in turn to a thickness of about 1000 .ANG., 3000 .ANG., and 5000 .ANG., respectively. They are then etched using the mask pattern 3 for a gate line, to form a semiconductor layer 15, a gate insulation film 16 and a gate electrode 17, thereby forming a stagger TFT-LCD.
According to the conventional method, a stagger TFT-LCD is fabricated with a simplified process using three mask pattern for cost reduction purposes. However, the TFT-LCD does not maintain the voltage applied to a pixel electrode through the TFT for a constant period of time, thereby causing problems such as a residual image.
In order to solve the problem of a residual image, a TFT-LCD has been proposed having a storage capacitor which maintains the voltage applied to a pixel electrode through the TFT for a constant period of time. However, since the process using three mask patterns or more should be carried out in order to fabricate the TFT-LCD adapting the storage capacitor, it has disadvantage in that the process is complicated and the cost of fabrication is high.